In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a field-induced photo-reactive alignment (FPA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type based on a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The liquid crystal display device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving characteristics of the composition. Table 1 below summarizes a relationship in two characteristics. The characteristics of the composition will be further described based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher, and a preferred minimum temperature of the nematic phase is about −10° C. or lower. Viscosity of the composition relates to a response time in the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, small viscosity in the composition is preferred. Small viscosity at a low temperature is further preferred. An elastic constant of the composition relates to a contrast ratio of the device. In order to improve the contrast ratio of the device, a large elastic constant in the composition is further preferred.
TABLE 1Characteristics of Composition and AM DeviceNo.Characteristics of CompositionCharacteristics of AM Device1Wide temperature range of aWide usable temperature rangenematic phase2Small viscosityShort response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage, smalldielectric anisotropyelectric power consumption andlarge contrast ratio5Large specific resistanceLarge voltage holding ratioand large contrast ratio6High stability to ultraviolet lightLong service lifeand heat7Large elastic constantLarge contrast ratio andshort response time
Optical anisotropy of the composition relates to a contrast ratio in the device. According to a mode of the device, large optical anisotropy or small optical anisotropy, more specifically, suitable optical anisotropy is required. A product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. In a device having a mode such as TN, a suitable value is about 0.45 micrometer. In the above case, a composition having the large optical anisotropy is preferred for a device having a small cell gap. Large dielectric anisotropy in the composition contributes to a low threshold voltage, small electric power consumption and a large contrast ratio in the device. Accordingly, the large dielectric anisotropy is preferred. Large specific resistance in the composition contributes to a large voltage holding ratio and the large contrast ratio in the device. Accordingly, a composition having the large specific resistance at room temperature and also at a temperature close to a maximum temperature of the nematic phase in an initial stage is preferred. The composition having the large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the device. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector, a liquid crystal television and so forth.
Vertical alignment of liquid crystal molecules is achieved by a specific polyimide alignment film in a general-purpose liquid crystal display device. In a liquid crystal display device having a polymer sustained alignment (PSA) mode, the alignment film is combined with a polymer. First, a composition to which a small amount of a polymerizable compound is added is injected into the device. Then, the composition is irradiated with ultraviolet light while voltage is applied between substrates of the device. The polymerizable compound is polymerized to form a network structure of the polymer in the composition. In the composition, alignment of liquid crystal molecules can be controlled by the polymer, and therefore the response time of the device is shortened and also image persistence is improved. Such an effect of the polymer can be expected for a device having the mode such as the TN mode, the ECB mode, the OCB mode, the IPS mode, the VA mode, the FFS mode and the FPA mode.
Meanwhile, in a liquid crystal display device having no alignment film, a liquid crystal composition containing a polymer and a polar compound is used. First, a composition to which a small amount of a polymerizable compound and a small amount of the polar compound are added is injected into the device. Here, the polar compound is adsorbed on a substrate surface and is arranged. The liquid crystal molecules are aligned according to the above arrangement. Then, the composition is irradiated with ultraviolet light while voltage is applied between substrates of the device. Here, the polymerizable compound is polymerized to stabilize the alignment of the liquid crystal molecules. In the composition, the alignment of the liquid crystal molecules can be controlled by the polymer and the polar compound, and therefore the response time of the device is shortened and image persistence is improved. Further, in a device having no alignment film, a process of forming the alignment film is unnecessary. No alignment film is applied thereto, and therefore no reduction of electric resistance of the device is caused by interaction between the alignment film and the composition. Such an effect caused by a combination of the polymer and the polar compound can be expected for a device having the mode such as the TN mode, the ECB mode, the OCB mode, the IPS mode, the VA mode, the FFS mode and the FPA mode.
A composition having positive dielectric anisotropy is used in an AM device having the TN mode. A composition having negative dielectric anisotropy is used in an AM device having the VA mode. A composition having the positive or negative dielectric anisotropy is used in an AM device having the IPS mode or the FFS mode. A composition having positive or negative dielectric anisotropy is used for an AM device having the polymer sustained alignment mode. In a device having no alignment film, a composition having positive or negative dielectric anisotropy is used. Examples of the liquid crystal composition having the positive dielectric anisotropy are disclosed in Patent literature Nos. 1 to 4 described below, or the like.